280 GEOLOGY. 



sponges, and the Protozoa, by rhizopods. This is a fine array for a 

 first appearance. As we come to study the representatives in more 

 detail, the breadth of the divergence, and the advance in evolution 

 that had already taken place, will appear still more impressive. These 

 representatives are all marine forms. Of land animals there are no 

 traces; but this negative record does not warrant the assertion that no 

 land animals lived at this time. Terrestrial arthropods, in the form of 

 scorpions and insects, appear in the record two periods later (Silurian), 

 and they were then rather highly developed, which renders an ancestry 

 reaching back as far as the Cambrian, or beyond, not improbable, 

 though by no means certain. 



No traces of vertebrates have yet been detected in the Cambrian 

 beds, but as fish remains have been found in the rocks of the following 

 period (Ordovician), it would be hazardous to assume that the lower 

 aquatic vertebrates were not among the denizens of the Cambrian 

 waters. There is no reason, however, to think that any of the terrestrial 

 vertebrates were present. 



The Cambrian Arthropoda. 1 — Of the four divisions of the Arthro- 

 poda, insects, spiders, myriapods, and crustaceans, only the last has 

 been found in the Cambrian strata. Its representatives were trilobites 

 and entomostraceans. 



1 The State and Government Reports are the chief media for the description of 

 fossils. Many descriptions are also found in scientific journals, and in the proceed- 

 ings of scientific societies, and, more rarely, in private memoirs. The literature, like 

 that on stratigraphy, is very voluminous, and cannot be fully cited in a work of 

 this kind. A convenient key to most of the recent literature is found in Weeks's 

 Bibliography and Index of North American Geology, Paleontology, Petrology, and 

 Mineralogy for 1892-1903, Bulls. 188, 189, 203, and 221, U. S. Geol. Surv., which will 

 probably be carried to later dates. References to earlier descriptions will be found 

 in Bull. 127, U. S. Geol. Surv. 



Some of the leading writers on American Cambrian fossils are: James Hall, Paleo. 

 N. Y., Vol. I, 1847, and Ann. Repts. N. Y., 1847-1863— notably the 16th; E. Bill- 

 ings, Paleozoic Fossils, Geol. Surv. of Canada, 1874; J. W. Salter, Quar. Jour. Geol. 

 Soc, 1859; S. W. Ford, Am. Jour. Sci., 1871-1881; R. P. Whitfield, N. Y. Repts., 

 Surv. of 40th ParaUel, and Wis. Repts., 1873-1879; C. Rominger, Proc. Phil. Acad. 

 Sci., 1887; J. F. Whiteaves, Am. Jour. Sci., 1878, and Canadian Rec. Sci., 1892; A. F. 

 Foerste, Bull. Mus. Comp. Zool., Cambridge, 1888, and Am. Jour. Sci., 1893; G. F. 

 Matthew, Trans. Roy. Soc. Canada, 1897-1900, Trans. N. Y. Acad. Sci., 1893-96, Bull. 

 Nat. Hist. Soc. New Brunswick, 1893-1902, and Canadian Rec. Sci., 1889-1894, and, 

 notably, CD. Walcott, Bulls. 10, 30, and 81, U. S. Geol. Surv., 10th Ann. Rept., U. S. 

 Geol. Surv., Am. Jour. Sci., 1887-88, and Proc. U. S. National Museum, Vols. XI-XIIL 

 The identifications of Walcott are chiefly followed in this work. 



